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Low intensity transcranial electrical stimulation (TES) in humans, encompassing transcranial direct current (tDCS), transcutaneous spinal Direct Current Stimulation (tsDCS), transcranial alternating current (tACS), and transcranial random noise (tRNS) stimulation or their combinations, appears to be safe. No serious adverse events (SAEs) have been reported so far in over 18,000 sessions administered to healthy subjects, neurological and psychiatric patients, as summarized here. Moderate adverse events (AEs), as defined by the necessity to intervene, are rare, and include skin burns with tDCS due to suboptimal electrode-skin contact. Very rarely mania or hypomania was induced in patients with depression (11 documented cases), yet a causal relationship is difficult to prove because of the low incidence rate and limited numbers of subjects in controlled trials. Mild AEs (MAEs) include headache and fatigue following stimulation as well as prickling and burning sensations occurring during tDCS at peak-to-baseline intensities of 1–2 mA and during tACS at higher peak-to-peak intensities above 2 mA. The prevalence of published AEs is different in studies specifically assessing AEs vs. those not assessing them, being higher in the former. AEs are frequently reported by individuals receiving placebo stimulation. The profile of AEs in terms of frequency, magnitude and type is comparable in healthy and clinical populations, and this is also the case for more vulnerable populations, such as children, elderly persons, or pregnant women. Combined interventions (e.g., co-application of drugs, electrophysiological measurements, neuroimaging) were not associated with further safety issues. Safety is established for low-intensity ‘conventional’ TES defined as <4 mA, up to 60 min duration per day. Animal studies and modeling evidence indicate that brain injury could occur at predicted current densities in the brain of 6.3–13 A/m 2 that are over an order of magnitude above those produced by tDCS in humans. Using AC stimulation fewer AEs were reported compared to DC. In specific paradigms with amplitudes of up to 10 mA, frequencies in the kHz range appear to be safe. In this paper we provide structured interviews and recommend their use in future controlled studies, in particular when trying to extend the parameters applied. We also discuss recent regulatory issues, reporting practices and ethical issues. These recommendations achieved consensus in a meeting, which took place in Göttingen, Germany, on September 6–7, 2016 and were refined thereafter by email correspondence. [ABSTRACT FROM AUTHOR]

Introduction Multiple studies indicated the left and right prefrontal cortex (PFC) and posterior parietal cortex (PPC) as anatomical substrates of the spatial working memory (i.e. frontoparietal memory network), however, the exact functional connectivity between these areas remains uncertain. This is partly due to the limitations of the applied neuroimaging methodologies, which are correlative in nature, as well as the classical brain stimulation approaches that are mostly operate within the limited number of neocortical sites. Objectives Here, we are taking advantages of the novel multichannel transcranial alternating current stimulation (tACS) method in order to investigate the role of phase connectivity between the frontoparietal regions in the left and right hemispheres that are commonly associated with the visual-spatial working memory related behavior. Materials & methods Fifteen healthy volunteers (11 females, age range: 19–28 years) participated in this double-blinded, placebo-controlled, randomized, crossover study. Each of them completed a 2-back visual-spatial working memory test during tACS and sham stimulation. Multichannel stimulation was applied at an intensity of 1 mA peak-to-baseline with a frequency of 6 Hz over the left PFC and PPC versus the right PFC and PPC. The working memory performance was estimated according to the signal detection theory as follows: d ′ = Z (“hit rate”) − Z (“false alarm rate”); and compared using the paired t-test. Results We observed the significant decrease in working memory performance during the desynchronization of the left and right frontoparietal regions in comparison to the control: d ′ control = 2.58 ± 0.12 (mean ± s.e.m.) and d ′ desynch = 2.34 ± 0.11 ( T (14) = 2.44, p = 0.03). No significant differences in the skin sensations and phosphenes were perceived according to the post-session written reports. Conclusion The efficiency of visual-spatial working memory in humans depends on the precisely calibrated phase connectivity within the frontoparietal network. Desynchronization of these relationships leads to the significant impairment of working memory performance. [ABSTRACT FROM AUTHOR]

Objective To investigate the role of the theta activity in the left DLPFC on visual-spatial working memory. Methods Fifteen healthy right-handed volunteers (9 females, age range: 20–26 years) were participated in this randomized, placebo-controlled, crossover study. Each of them completed a 2-back match-to-sample visual-spatial memory test during transcranial alternating current stimulation (tACS; 10 min, 1 mA peak-to-baseline, 6 Hz) and sham stimulation. A focal Laplacian electrode-montage with 5 electrodes was used for stimulation (3 cm 2 each, the central electrode at AF3). Before and after each session EEG was recorded. Results We observed a significant increase of the hit rate in the memory test during stimulation (Hit control = 68.8 ± 3.2%, Hit stimulation = 73.1 ± 3.5%; p = 0.007, g = 0.33 (Paired t -test and Hedges’ g statistics)), but no effect on the false alarm rate (FA control = 19.8 ± 2.2%, FA stimulation = 18.5 ± 2.5%; p = 0.55). EEG analysis revealed an increase of the local theta band power after the stimulation ( p = 0.048, g = 0.58). Conclusions tACS at 6 Hz over the left DLPFC increases the theta power and improves the hit rate in the 2-back visual-spatial working memory test. Key message Theta oscillations in the left DLPFC affect the spatial working memory accuracy. [ABSTRACT FROM AUTHOR]